Glossary

When you’re thinking about spending money on a new piece of equipment, you want to be sure you understand what you’re spending your money on. Below are some common terms you will run across while comparing and reading about air compressors:

Air Filter – Tucked inside the inlet head is the air filter, usually made of paper but sometimes also of pleated cellulose, polyester, or glass. They filter particles down to 2 microns, ensuring that what floats around your garage or shop won’t clog up the compressor or hose.

Check Valve – One-way valves used to make sure air is properly compressed, to prevent air in the tank from blowing out through the compressor, and to aid in pressure regulation after the compressor is shut off.

CFM – Cubic Feet per Minute, a unit of volumetric capacity, calculated at the inlet of the air compressor. It can be divided into Standard CFM, where air is being moved at “standardized” conditions, or Actual CFM, which depends on the atmospheric pressure, the ambient temperature, and area humidity.

Drain Valve – When air is compressed and stored inside the air compressor tank, water vapor in the air condenses and precipitates, and pools at the bottom of the tank. The drain valve is used to drain this condensate out so the tank does not rust or otherwise become damaged.

Duty Cycle – The percentage of time an air compressor can run continuously before it needs to rest. An air compressor with a 50% duty cycle can run for 10 minutes, for example, then will need to rest for ten minutes before running again.

Horsepower (HP) – Simply put, horsepower measures the amount of power a piece of machinery (like the motor in an air compressor) delivers. A higher horsepower rating in compressors normally equates to a higher CFM rating. Avoid cheap air compressors with high horsepower but low CFM as that indicates they are inefficient.

Oil-Free Compressor – Also referred to as oil-less compressors, they are designed to require minimal maintenance at the expense of lifespan. While they typically cost less than models that use oil, most are not recommended if you plan on anything more than just occasional use.

Pneumatic – Containing or operating under the power of air or gas under pressure. All air compressors, and all tools powered by such, are pneumatic.

PSI– One pound of force per square inch. It is the measure of pressure in the avoirdupois system. For example, the atmosphere is roughly 14.69 PSI. For air compressors, a higher maximum PSI is not always better.

Pressure Gauge – Absolutely vital on an air compressor, this tells you how much air is built up inside the tank, reading in PSI and kiloPascals usually.

Pressure Switch – The “On/Off” or “Auto/Off” switch, it runs continuously when turned on until it reaches a pre-set pressure, at which time it stops compressing.

Single-Stage Compressor – Most air compressors are single-stage compressors, in that they only compress the air once before storing it in the tank to be used. 2-stage compressors exist which are much more efficient but also much more expensive in most cases.

Regulator – Used for fine-tuning the amount of pressure you need for the tool you are using, regulators decrease the outlet PSI to a more manageable level which most air tools require.

Tank Capacity – It’s important to keep in mind what you will be using your air compressor for, and how often you will be using it at a time. This will inform you how much tank capacity you need; the more you will use compressed air, or the greater your CFM needs, the more capacity you require.